In order to enable the use of a filter medium which does not posses sufficient rigidity to be self-supporting, a roll type filter apparatus has been provided in which an endless flexible mesh grid is caused to travel through the filter chamber and the filter medium comprises a web of filter material deposited on the mesh grid and carried by the grid through the filter chamber. The supporting grid is in the form of an endless band which recirculates through the filter chamber, for example in a zig zag path between two series of guide rolls extending respectively across the gas inlet and gas outlet sides of this filter chamber. The filter web or mat is fed into the filter chamber onto the supporting grid at one end and is withdrawn from the other end. In this fashion the gaseous flow through the filter chamber is caused to flow through the filter mat deposited on the mesh grid thereby filtering out particulate material from the gaseous flow.
With filter apparatus of this character, there is a tendency, particularly when the filter medium is dense and offers a high resistence to gaseous flow through it, for the gaseous medium to pass around the edges of the filter medium between the side walls of the filter chamber and the side edges of the filter medium along the marginal portions of the endless grid.
Prior to the present invention, the problem of leakage of dirty air around the edges of the filter medium has been addressed by the provision of a special trackway in the side edges of the filter chamber which is designed to accomodate the filter medium as it is advanced through the filter chamber. For example, as shown in U.S. Pat. No. 3,280,538, the construction of the trackway for the filter medium is effective if the filter medium is readily compressible so that the track arrangement may compressively engage the edges of the filter medium and provide a seal against the escape of gaseous medium around the edges. Such an arrangement not only requires substantial thickness and compressability in the filter web, but also requires that the filter web accurately track within the guideways so that lateral displacement of the web does not effect disengagement of the web from the guideways. Where the filter web is thin and flexible and is not possessed of a high degree of compressability, the arrangement shown in this patent is not satisfactory.
A similar arrangement is shown in U.S. Pat. No. 3,552,099 wherein the filter web is carried by a wire mesh conveyor having an edge form which rides in a track along the side walls of the filter chamber. A sealing flap overlies the track to engage against the exposed surface of the filter web on the wire conveyor to seal against the web and prevent leakage of gaseous medium between the web and the side wall. Where the web material is not resistant to abrasion, such a sealing arrangement tends to cause separation of the web material and enables leakage through the separations produced.
It has also been proposed as shown in U.S. Pat. No. 3,406,503 to utilize a filter blanket having an edge construction formed to travel in a guideway in the side walls of the filter chamber, the edge construction either being an integral part of the filter blanket or being a strip applied to the edge of the filter material and having a beaded construction adapted to ride in the guideways. Such an arrangement requires the filter material to be designed specifically to accomodate to the trackway arrangement in the filter chamber and substantially increases the cost of the filter material.